Chemical recycling of mixed thermoplastics via pyrolysis: A comparative study of feedstock influence and reactor impact

Pyrolysis represents a promising solution for increasing plastic waste recycling rates in a circular economy. The thermal decomposition behavior of pure polymers is well investigated. However, the diverse composition and complex technical reactor conditions pose challenges for the pyrolysis of heterogeneous plastic waste. Therefore, this study examines the pyrolysis of virgin polymers, defined reference mixtures, and post-consumer waste in a scalable stirred tank reactor (STR) compared to an auger-type screw reactor (ASR) at pilot-scale. The results of the STR confirm that simple polymer blends of polyolefins and polystyrene degrade following the decomposition mechanisms of the pure polymer, while heteroatom-containing mixtures exhibit interaction effects. These interactions shift the products from the condensable to the gaseous fraction by up to 22 wt.%. Furthermore, product yields and compositions strongly depend on reactor design and process parameters. The gas residence time and the time-dependent polymer pyrolysis temperature are identified as the main influencing parameters. In this study, interaction effects between different polymers overlap the reactor-specific impact in complex feedstock mixtures, dominating the product yields. The condensate yield in the STR exceeds the results obtained in the ASR by 10-24 wt.% for LDPE, PP, PS, and a mixture of them. In contrast, the STR system yields only up to 6 wt.% more condensates than the ASR system for mixtures containing oxygen, nitrogen, and chlorine. The reactor type has a major impact on condensate quality. The H/C ratio, heteroatom content, and proportion of distillation cuts in the light and middle distillate range vary significantly. • Pyrolysis of mixed thermoplastics is compared for stirred-tank and screw reactors • Blends of polyolefins and polystyrene decompose following pure polymer behavior • Interactions in heteroatom-rich mixtures additionally increase gas yields by up to 22 m.% • Reactor type-dependent characteristics affect the yields and the product composition • Polymer interactions outweigh reactor-specific effects in complex polymer mixtures